1. Field of the Invention
The present invention relates to a thin-film transistor having thin semiconductor films formed on an insulating substrate, and also to a method of manufacturing the thin-film transistor.
2. Description of the Related Art
Thin-film transistors (TFTs), each comprising various thin films of a predetermined shape which are formed, one upon another, on an insulating substrate, are used as active elements in an active matrix liquid-crystal display. The active elements of the liquid-crystal display of this type are reverse stagger type TFTs.
As is shown in FIG. 5, the reverse stagger type TFT comprises a gate electrode 2 formed on the insulating substrate made of glass or the like, a gate insulating film 3 made of SiN (silicon nitride) and covering the gate electrode 2, an i-type semiconductor layer 4 made of a-Si (amorphous silicon) and formed on the gate insulating film 3 and opposing the gate electrode 2, an n-type semiconductor layer 5 made of a-Si doped with an n-type impurity and formed on the i-type semiconductor layer 4, and a source electrode 6a and a drain electrode 6b, both formed on the n-type semiconductor layer 5. A part of the n-type semiconductor layer 5 has been etched away, thus exposing the channel-forming region of the i-type semiconductor layer 4 and which is located below the gap between the source electrode 6a and the drain electrode 6b.
The thin-film transistor shown in FIG. 5 is manufactured in the following method. First, the gate electrode 2 is formed on the insulating substrate 1. Then, the gate-insulating film 3, the i-type semiconductor layer 4, and the n-type semiconductor layer 5 are formed, one upon another in the order mentioned, on the substrate 1 and the gate electrode 2. The source electrode 6a and the drain electrode 6b are formed, spaced apart from each other, on the n-type semiconductor layer 5. Thereafter, said part of the n-type semiconductor layer 5 has been etched away, thus exposing the channel-forming region of the i-type semiconductor layer 4 which is located below the gap between the electrodes 6a and 6b. The n-type semiconductor layer 5 should not contact the channel-forming region. If the layer 5 contacts the channel-forming region, the surface of the i-type semiconductor layer 4 will inevitably be etched and damaged when said part of the layer 5 is etched away.
To prevent such damage to the i-type semiconductor layer 4, a blocking film 7 is formed on the channel-forming region of the i-type semiconductor layer 4 as is shown in FIG. 5. Hitherto, the blocking film 7 is made of SiN. The blocking film is formed in the following way. After the i-type semiconductor layer 4 has been formed on the gate insulating film 3 covering the gate electrode 2, an SiN film is deposited on the i-type semiconductor layer 4 by means of plasma CVD method. Next, the SiN film is patterned, forming the blocking film 7 large enough to cover the entire channel-forming region. Then, the n-type semiconductor layer 5, the source electrode 6a, and the drain electrode 6b are formed, and that part of the layer 5 which is located on the blocking film 7 is etched away, in the method specified above.
Since the blocking film 7, thus formed, covers the channel-forming region of the i-type semiconductor layer 5, it protects the channel-forming region from damage which may be done during the etching of the n-type semiconductor layer 5.
The conventional method of manufacturing a thin-film transistor is disadvantageous, however, because the blocking film 7 is made of SiN. When the SiN film is patterned to form the blocking film 7, defects such as pinholes may be formed in the gate insulating film 4 which is located below the i-type semiconductor layer 5. The etchant used to pattern the SiN film flows through pinholes, if any in the i-type semiconductor layer 4, reaching the gate insulating film 3. The etchant inevitably etches the gate insulating film 3, forming defects, such as pinholes, also in the gate insulating film 3.
Pinholes, if made in the gate insulating film 3, cause interlayer short-circuiting between the gate electrode 2, on the one hand, and the source electrode 6a and the drain electrode 6b, on the other. This is because the n-type semiconductor layer 5 and the electrodes 6a and 6b are formed after the blocking film 7 has been formed by patterning the SiN film. Interlayer short-circuiting may take place, not only between the gate electrode 2 and the electrodes 6a and 6b, but also between the gate line (not shown) formed integral with the gate electrode 2 and the data line (not shown) formed integral with the source electrode 6a and or the data line integral with the drain electrode 6b, or between the data lines integral with the electrodes 6a and 6b.
Therefore, it is quite probable that the thin-film transistor can be manufactured by the conventional method, but at a low yield.